A top federal safety regulator tasked with handling the U.S. government's response to a string of oil-train crashes in recent years is stepping down, agency officials said Wednesday.

Cynthia Quarterman,
administrator of the Pipeline and Hazardous Materials Safety Administration, which is part of the Transportation Department, will leave on Oct. 3, the officials said.

In an email to staff, Ms. Quarterman praised the work the agency has done under her leadership, but didn't say why she was leaving.

"We have both closed regulatory gaps and removed unnecessary regulatory hurdles," "We have held regulated industries accountable through strengthened oversight and enforcement, but also improved outreach and training to them, the public and the response community," Ms. Quarterman said.

The agency declined to comment beyond the email.

Ms. Quarterman assumed the post in 2009 and it wasn't immediately clear why she was resigning. Her position requires Senate confirmation, and it wasn't immediately clear who President
Barack Obama
would nominate to take her place.

The mission of the safety agency, known as the PHMSA, is to prevent and respond to risks and accidents posed by transporting hazardous materials via pipelines and railways.

Both Ms. Quarterman's agency and the Obama administration more broadly have faced criticism from those who contend that they haven't sufficiently regulated trains carrying crude oil. Between 2011 and the first part of 2014, rail deliveries of oil more than doubled, according to U.S. Energy Information Administration data.

PHMSA has also faced criticism from its internal watchdog for not ensuring that states enforce safety rules on natural-gas pipelines. The Transportation Department's inspector general reported in May that lapses at PHMSA have resulted in "undetected safety weaknesses" in state oversight programs.

Concerns about the safety of decades-old pipelines have been on the rise following a series of fatal pipeline explosions, including one in San Bruno, Calif., in September 2010 that killed eight people. More recently, a gas explosion in New York City in March left eight people dead.

Earlier in her career, Ms. Quarterman was the nation's top oil and natural gas regulator as director of the then-Minerals Management Service during the Clinton administration. After the 2010
BP
oil spill, that Interior Department agency was broken into separate bureaus.